Rheology includes the study of the viscosity in fluid systems and its potential dependence on magnitudes such as the fluid velocity. Usually, macroscopic rheometers are used to measure the viscosity of liquids and suspensions, specifically for those fluids which are characterized by a viscosity which depends upon the values of the velocity of the fluid or the applied pressure.
There is a growing necessity to develop micro-rheometers that take the advantages associated to miniaturized systems: lower volume material, shorter experimental measurements, reduction of costs, and small and portables devices.
Currently, these micro-devices are used as biological or biomedical instruments for sample monitoring (e.g. sugar solutions) and/or medical detection of anomalies (e.g. fluids from knee joints) [Hennemeyer, Marc et al. “Cantilever Micro-rheometer for the characterization of sugar solutions”, Sensors 2008. 8, 10-22; Sandia National Laboratories (2012 Sep. 26) “Students painlessly measure knee joint fluids in annual Sandia contest”, retrieved from https://share.sandia.gov; Ziemann, F et al. “Local Measurements of Viscoelastic Moduli of entangled actin networks using an oscillating magnetic bead micro-rheometer”, biophysical Journal, 1994, Vol. 66, 2210-16]. Furthermore, there are several diseases associated to blood disorders such as leukaemia, cardiovascular disease, sickle cells anaemia, kidney disease, polycythemia, anaemia, etc, that alter the rheological properties of blood, in particular its viscosity.
Several techniques are currently used to develop these micro-biotechnologies, for instance, particle tracking methods where paramagnetic particles are suspended in a viscous fluid. The particles are optically tracked while they are attracted by an external magnetic force. The rheological properties of the fluid are then determined in terms of the behaviour of the tracer particles [previous article of Ziemann, F et al.; Song, Jin-Oh et al. “Magnetic microrheometer for in situ characterization of coating viscosity”, Rev. Sci. Instrum., 2010, Vol. 81, 93903 1-8].
More complex micro-electro-mechanical systems (MEMS) are used to study the rheology of fluids. One type of such devices consists of two plates where the fluid sample is placed between them. One plate is fixed while the other plate, controlled by a thermal actuator, moves up and down to deform the sample at different frequencies and measure viscoelastic moduli [Christopher, Gordon F. et al. “Development of a MEMS based dynamic rheometer”, Lab Chip, 2010, Vol. 10, 2749-57].
A common feature among most micro-devices for rheological applications is that they are subject to complex analysis, expensive fabrication and/or bulky and expensive equipment to measure the viscosity of fluids.
Moreover, conventional macroscopic rheometers are limited by a minimum torque value that limits the viscosity measurement of fluids of low viscosity values at very low shear rates (or velocities), having as a requirement to perform the measure at higher shears in order to get a valid viscosity result.
Several patent documents refer to rheometers. Patent document US20080134765-A1 discloses a micro-rheometer for measuring flow viscosity and elasticity for micron sample volumes using pressure sensor arrays. The pressure sensors measure the viscosity of the sample liquid while flowing in a uniform length of a flow passage, since from the pressure measurement wall shear stress can be calculated. As it is acknowledged in this patent document, using pressure sensors has several drawbacks, since a perturbation of flow significantly influences pressure measurement, in particular for non-Newtonian liquids. Moreover, any slight surface roughness due to the mounting of pressure sensors may be a source of test sample deposition, which degrades long term performance of the micro-rheometer. There are also some difficulties when mounting individual pressure sensors. Therefore, the measurement accuracy is often compromised depending on how well the individual pressure sensors are mounted in the flow channel. To overcome the problem of patterned structures on flow channel surfaces caused by placement of individual pressure sensors in the flow channel, US20080134765-A1 discloses the use of pressure sensors monolithically integrated into the wall of the flow channel.
However, all these problems are not satisfactorily solved, as explained in patent document GB2485965, which refers to a rheometer for measuring the viscosity and elasticity of liquids. This patent document explains that the previous patent document, US20080134765-A1, embeds the pressure sensors in the body of material defining the flow channel with the aim to ensure that the flow of fluid within the channel is not disturbed by the pressure sensors, in order to obtain a smooth, uninterrupted internal surface of the channel. However, it acknowledges several disadvantages associated with the integrated pressure sensor arrangement:                The apparatus is difficult and expensive to produce.        Once the arrangement has been manufactured there is no flexibility in where to locate the pressure sensors.        The separation of the pressure sensors from the flowing liquid by even a small thickness of material can reduce the accuracy of the pressure measurements obtained from the sample.        It only measures steady state viscosity of sample liquids.        
Other patent documents, such as DE102006001180-A1 and FR2510257-A1, employ optical sensors to measure the flow velocity. The disadvantages of these optical approaches rely on integration problems (mismatches) with the coupling of the optical elements and the microscale's fluidic device as well as the use of complex electronic control methods to assure the proper operation and data management of the optical sensors.
Patent document EP1923707-A2 discloses a micro-rheometer with a microchannel and a sensor array arranged to measure rheological properties of a fluid, wherein the sensor array comprises a plurality of pairs of electrodes, the two electrodes of each pair being placed face to face within the microchannel to function as an electronic switch when the fluid flows through them.
Patent document US2010/0042339-A1 discloses a fluidic analysis device for determining characteristics of a fluid, with at least one flow channel, means for directing a fluid into the flow channel, and at least two analytical means suitable for analyzing a sample.
The present invention proposes a different approach to measure the viscosity of Newtonian and non-Newtonian fluids that overcomes some of the above-mentioned problems.